Process for preparing phthalic anhydride

ABSTRACT

Phthalic anhydride is prepared by the oxidation of ortho-xylene with molecular oxygen at a temperature of about 200° to about 600° C. in the presence of an oxidation catalyst that comprises an essentially inert support having a strongly-adhering coat of a catalytic composition containing an oxide of vanadium wherein the concentration of vanadium is less than 10%.

REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part to our earlier application, Ser. No.572,260, filed Apr. 28, 1975, now abandoned, which is a continuation ofSer. No. 362,412, filed May 21, 1973, now abandoned.

BACKGROUND OF THE INVENTION

The oxidation of ortho-xylene to phthalic anhydride is known. Thereactant ratios, conditions and other parameters shown in the art aresubstantially unchanged. The present invention is the discovery of aparticular group of catalysts that are especially effective in thisreaction. The catalysts are very easy to prepare; they give smoothtemperature control and high per pass conversions.

U.S. Pat. No. 3,464,930 discloses use of a catalyst containing vanadiumand titanium coated on nonporous, inert materials, i.e., materials freefrom pores or having a small surface area, preferably less than 3 sq.m./g. This patent discloses the preparation of a catalyst containing theoxides of titanium and vanadium by pretreating a carrier of fusedaluminum oxide with titanium tetrachloride and glacial acetic acid andbringing this carrier in contact with a paste comprising vanadiumoxalate and anatase.

British Pat. No. 1,140,264 is an equivalent of U.S. Pat. No. 3,464,930.

British Pat. No. 1,203,321, a Patent of Addition to No. 1,140,264,discloses oxidation catalysts which are an improvement over theoxidation catalysts containing vanadium pentoxide and titanium dioxidein British Pat. No. 1,140,264. U.S. Pat. No. 1,203,321 discloses thatthe coating on catalysts which have been coated with the compositioncontaining vanadium pentoxide and titanium dioxide by applying thecomposition in a dissolved or suspended form to a carrier which has beenheated to a temperature of 160° to 500° C. has greater strength. Theobject of this invention is to overcome the disadvantage in British Pat.No. 1,140,264 of resistance to abrasion.

British Pat. No. 1,237,954 discloses the air oxidation of o-xylene in atubular reactor at an elevated temperature in contact with a fixed-bedcatalyst comprising a nonporous carrier having a coating of acomposition containing vanadium oxide and titanium oxide. The object ofthis invention is to overcome the disadvantage of catalyst discolorationin British Pat. No. 1,140,264.

British Pat. No. 1,238,379 discloses use of supported catalystscontaining titanium dioxide, vanadium pentoxide and aluminum oxide,lithium oxide and/or zirconium dioxide coated on an inert nonporouscarrier. The object of this invention is to overcome the disadvantage inBritish Pat. No. 1,140,264, wherein the catalyst does not achieve theiroptimum effect until after a certain operational period.

British Pat. No. 1,267,043 discloses the use of catalysts containingvanadium pentoxide, titanium dioxide and a phosphorus compound coated onnonporous inert carriers. The object of this invention is to improve thelife of catalysts such as those described in British Pat. No. 1,140,264.

Only unexpected results are obtained using novel catalysts of theinvention for the production of phthalic anhydride.

SUMMARY OF THE INVENTION

The present invention is the discovery of particular catalysts that areespecially effective in the oxidation of ortho-xylene to phthalicanhydride. In the process for preparing phthalic anhydride by contactinga mixture of ortho-xylene and molecular oxygen at a temperature of about200° to about 600° C. in the presence of an oxidation catalyst which isuseful for reaction in a fixed-bed reactor, the reaction is improved byusing an oxidation catalyst comprising (a) an essentially inert, atleast partially porous support having a particle size of at least about20 microns, said inert support having an outer surface; and (b) acoating of a catalytically active material on said outer surface of saidsupport which strongly adheres to said outer surface of said inertsupport, said catalytically active material containing boron, antimonyor mixture thereof and an oxide of vanadium such that the weight ofvanadium is less than 10% of the total weight of the oxidation catalyst,and wherein the active catalytic material optionally contains silica.The catalysts of the present invention give a controllable reaction withhigh per pass conversions to phthalic anhydride and desirably lowamounts of useless byproducts. The catalysts are also especially easy toprepare and are attrition resistant.

As noted, the focal point of the present invention is the use of a newcatalyst in the known oxidation of ortho-xylene. This catalyst givesespecially good results in the reaction.

The special coated catalyst consists of an inner-support material havingan outer surface and a uniform coating of the active catalytic materialon this outer surface.

The essentially inert support can be selected from any material that isat least partially porous and that has the physical integrity towithstand the catalyst preparation techniques and the conditions of theoxidation reaction. The porosity is required for the preparation of thecatalyst, and the stability of the support is required for the attritionresistance.

By the preferred procedure of the invention, the support materialemployed is at least partially porous. By this is meant the supportmaterial must be susceptible to the penetration of liquid. Preferredsupport materials are capable of absorbing at least 1% by weight waterbased upon the weight of the support.

Suitable materials that are among the inert supports include silica,Alundum, alumina, alumina-silica, silicon carbide, titania and zirconia.This list is representative of those materials that can be used. Ofcourse, many other materials having the attributes discussed above couldbe substituted for these support materials.

As noted in the broad description of the invention, the inert support isat least 20 microns in size. There is no theoretical upper limit to thesize of the inert support, but for practical reasons, the support isnormally not larger than about 5 centimeters. Preferred supports areabout 0.2 cm. to about 2 cm.

The shape of the inert support may vary widely. Irregular or regularshapes may be used, and in the preferred practice of the invention,spherical supports are employed.

Having described the inert support component of the catalyst, the activecatalytic material will be described. As noted, the active ingredientscontain at least an oxide of vanadium. Broadly preferred in the presentinvention is vanadium pentoxide that has been partially reduced with areducing agent so that at least some of the vanadium is present at alower valence state than +5.

The partial reduction of the vanadium in the catalyst is convenientlyaccomplished by reducing vanadium pentoxide with a suitable reducingagent. Representative examples of reducing agents include: organicreducing agents, such as hydrazine, or finely divided metals, such astungsten or molybdenum.

Specific catalyst compositions of special interest are described by theformula

    D.sub.b V.sub.12 O.sub.x

wherein

D is B, Sb or mixtures thereof;

b is a number from greater than 0 to 10; and

x is the number of oxygens required to satisfy the valence requirementsof the other elements present.

Of these catalysts, those containing a mixture of boron and antimony arepreferred. Most preferred are those wherein b is a number from greaterthan 0 to 5. Also preferred are catalysts wherein the catalyticallyactive material consists of a mixture of the active catalyticingredients and silica.

A very important aspect of the present invention is that the weight ofvanadium in the catalyst is less than about 10% by weight of the entirecatalyst. Preferred are catalysts that contain less than 5 percentvanadium, with the best results being obtained with those catalystscontaining less than 1 percent vanadium.

Even though the vanadium oxide could be placed on the inert support asthe pure oxide, it is preferred to mix the active catalytic ingredientswith a solid diluent prior to coating the inert support. Such soliddiluents may conveniently be any of the support materials used or anyother solid diluent. Preferred solid diluents are titanium dioxide,silica or mixture thereof. The solid diluent may be combined with theactive ingredients in any combination that will give the desiredcatalyst. Most preferred are catalysts employing a solid diluent andcontaining less than about 0.5% by weight of vanadium.

The total coated catalyst of the present invention is convenientlyprepared by partially wetting the inert support with a liquid such aswater. This partially wet support should contain some liquid, but thereshould be no surface liquid visible. The support should not be wet onthe outer surface of the total mass. It should be dry to the touch. Thepartially wet support is contacted with a powder of the activeingredient composition, and the inert support is rolled in the activeingredients. The contact between the powder and inert support is easilyaccomplished by placing the support in a closed container, rotating thecontainer in an inclined plane and adding portions of the powder.Preferably, substantially all of one portion of the powder is coated onthe support before another portion is added.

By the preferred procedure of the invention, the catalyst is prepared bycontacting an essentially inert, at least partially porous support of atleast 20 microns in diameter with a measured amount of liquid to producea partially wet support, said support being one that does not have theappearance of having liquid on the outer surface of the support, but hasat least some liquid absorbed on the support, and (2) rolling thesupport in a powder of the catalytically active material to produce asupport having a hard uniform coating of the catalytically activematerial on the outer surface of the support.

The catalysts prepared by this process consist of the inert support anda strongly-adhering coat of the active catalytic ingredients on theouter surface of the support. The catalytic ingredients are maintainedon the surface of the support, and there is essentially no impregnationof the active ingredients into the inert support. Thus, the catalysts ofthe invention are sharply contrasted with those catalyst techniques thatimpregnate an inert support with an active catalyst by contacting thesupport with a liquid or slurry of active ingredients.

The coated catalyst of the present invention is used in the oxidation ofortho-xylene to phthalic anhydride. This oxidation is a known reaction,and the reaction conditions, feed ratios and design of the reactionsystem is not materially changed from that of the art. Broadly, theratio of molecular oxygen to ortho-xylene could be as low as about fourmoles per mole of xylene, but there is no theoretical upper limit.Normally, the molecular oxygen is added as air, and the air/xylene ratiousually ranges from about 40 to 130 or more.

The reaction temperature may vary widely but is usually within the rangeof about 200° to about 600° C, with temperature of about 300° to 500° C.being preferred. The reaction can be run at atmospheric,superatmospheric or subatmospheric pressure. The catalysts of theinvention are most suited for fixed-bed operation, but using smallsupport particles, it is also possible to conduct the reaction in afluid-bed reactor.

The catalysts of the invention produce especially high yields ofphthalic anhydride and useful byproducts. The oxidation reaction, usingthe catalysts of the invention, is also easy to control. In the past,the exotherm created in the reactor caused loss of control, but now withthe catalysts of the invention, an even temperature is easily maintainedwithout use of special diluents or low reactant feed rates.

SPECIFIC EMBODIMENTS EXAMPLES 1-11 Preparation of Phthalic AnhydrideUsing Coated Catalysts of the Invention.

Various catalysts of the invention were prepared as described below. Acatalytic reactor having a reaction zone of 20 cc. was constructed froma 1.02 cm. inside diameter stainless steel tube. The reactor wasmaintained in a metal block furnace, and a thermocouple was placedinside of the catalytic bed. Ortho-xylene and air were fed through thecatalytic reactor to give the desired contact time. The conditions andresults of these experiments are shown in the Table. The percent perpass conversions are the gram-atoms of the carbon in the productspecified which are found in the effluent of the reactor times 100,divided by the gram-atoms of carbon fed as ortho-xylene.

The active catalytic ingredients were ground to a powder. Separately,the appropriate mesh size Alundum particles were wetted with water sothat the increase in weight over the dry particles was about 2 to 4%.There was no surface wetness on the Alundum. This material was rotatedin an inclined glass jar, and the appropriate gram portions of theactive catalytic ingredients were added. Essentially all of each portionof the active catalytic material was used up prior to the addition ofthe next portion. The catalysts were dried overnight at 110° C, andactivated in the reactor for two hours at 426° C.

The catalysts for the reactions shown were prepared as follows:

EXAMPLE 1 20.6% (Sb₂ B₃ V₁₂ O_(x) +W₁.32) and 79.4% Alundum

181.9 grams of vanadium pentoxide was slurried in about one liter ofwater. To this stirred slurry was added 40.4 grams of tungsten metalpowder. The slurry was allowed to boil and reflux for several hours,during which time the color changed to deep blue-black.

An aliquot of this dark-colored slurry, containing 0.507 gram atomvanadium was transferred to another beaker. To this was added 7.84 gramsof boric acid. This mixture was allowed to reflux for several hours.Finally, 12.3 grams of antimony trioxide was added and refluxingcontinued for several more hours. The final mixture was then evaporatedto a thick paste and then dried overnight at a temperature of 110° C.

The black powder obtained was coated onto an Alundum support (10-30 meshsize) by the following procedure. 25 grams of Norton SA 203 Alundum waswetted with 2 grams of water by rolling in a round glass jar. Thematerial was free-flowing and outwardly dry. To this material, in arotating jar, was added 12.5 grams of the dried black powder in severalincremental additions. The powder was evenly coated onto the surface ofthe Alundum. The final product was dried, and 6 grams of active powderwas recovered such that the final product consisted of 6.5 grams ofactive powder and 25 grams of Alundum which calculated to be 20.6%active and 79.4% Alundum support.

EXAMPLES 2 AND 3 9.7% (Sb₂ B₃ V₁₂ O_(x) +W°₀.6) and 90.3% Alundum

In a manner essentially identical to that of Example 1, a similarcatalyst was prepared with the following quantities of materials: 90.95grams vanadium pentoxide, 9.2 grams of tungsten metal powder, 15.46grams of boric acid, and 24.3 grams of antimony trioxide. The finalslurry was evaporated to a thick paste and dried as before.

This powder, screened to less than 50 mesh, was coated onto 10-30 meshAlundum in an analagous manner. After drying 9.8 grams of loose powderwas removed such that the composition of the finished material was 9.7%active coating and 90.3% Alundum support.

EXAMPLES 4 AND 5 5% (6B₃ V₁₂ O_(x).94TiO₂) and 95% Alundum

90.95 grams of vanadium pentoxide was slurried in 600 ml. of distilledwater, and to this was added 9.7 grams of 85% hydrazine hydrate. Theslurry starting darkening in color immediately. The slurry was refluxedfor several hours. After this, 15.46 grams of boric acid was added andrefluxing continued for several additional hours. Color changed fromdeep blue-black to dark green. This mixture was designated Mixture A.

An aliquot of Mixture A was removed such that the aliquot contained theequivalent of 6.26 grams of vanadium pentoxide. The aliquot was dilutedto 350 ml. with distilled water and 97 grams of titanium dioxide wasadded. This slurry was refluxed for one hour then allowed to evaporateto a thick paste, which was then dried overnight at 110° C.

The dried product above was screened through 50 mesh then coated asfollows: 30 grams of low porosity Norton BA 307 Alundum chips (10-20mesh) was pre-wet with 0.5 grams of water in a rotating round glass jar.The material was free-flowing and outwardly dry in appearance. To thiswas added three portions, consisting of 0.53 grams each, of thecatalytically active powder. The powder coated the Alundum uniformly.The coated material was dried overnight at 110° C. Final composition was5% active component-95% Alundum support.

EXAMPLES 6 AND 7 5% (6Sb₂ B₃ V₁₂ O_(x).94TiO₂) and 95% Alundum

To the remaining portion of Mixture A having a calculated quantity ofvanadium of 0.93 gram-atom was added 22.6 grams of antimony trioxide andthe mixture was refluxed for two hours. This mixture was then designatedMixture B.

An aliquot of Mixture B was removed with a calculated amount ofvanadium, calculated as vanadium pentoxide, equal to 5.57 grams. Thisaliquot was diluted to approximately 350 ml. with distilled water and87.2 grams of titanium dioxide was added. The resulting slurry wasstirred and then evaporated down to a thick paste, which was then driedovernight at 110° C. The dried material was crushed and screened through50 mesh.

This dried material was coated onto 10-20 mesh low porosity Norton BA307 Alundum chips in a manner analagous to that described above. 30grams of Alundum was used, and this was pre-wet with 1.1 grams of water.Total amount of dried active material coated amounted to 1.59 grams,added in three equal increments. The finished material consisted of 5%active coating and 95% Alundum support.

EXAMPLE 8 8% (Sb₂ B₃ V₁₃ O_(x)) and 92% Alundum

A portion of Mixture B was stirred and evaporated down to a thick pasteand dried overnight at 110° C. The dried material was ground andscreened through 50 mesh. This material was coated onto low porosityNorton BA 307 Alundum particles (10-20 mesh) in a manner analogous tothat described above. 30 grams of Alundum was used and this was pre-wetwith 1.2 grams of water. Five portions of active powder, consisting of0.53 grams each, were added during the coating operation. The coatedmaterial resulting was dried overnight at 110° C. The coating wasuniform with only a small amount of loose powder, thus the finalcomposition was about 8% active coating and 92% Alundum support.

EXAMPLE 9 5% (6Sb₂ B₃ V₁₂ O_(x).94ZrO₂ /CaO) and 95% Alundum

An aliquot of Mixture B containing 5.57 grams of vanadium pentoxide wasdiluted to about 300 ml. with distilled water. To this was added apowdered calcium oxide stabilized zirconia and the resulting slurry waswell stirred and evaporated down to a thick paste. This was driedovernight at 110° C. and then ground and screened through 50 mesh.

This material was coated using 30 grams of low porosity Norton BA 307Alundum which has been pre-wet with 1.0 gram of water. Three 0.53portions of the active catalytic powder was coated onto the Alundum. Thematerial was dried at 110° C. and consisted of 5% of the active powderand 95% Alundum support.

EXAMPLE 10 5% (6Sb₂ B₃ V₁₂ O_(x).94SiO₂) and 95% Alundum

An aliquot of Mixture B containing 5.57 grams of vanadium pentoxide wasdiluted to about 300 ml. with water. To this was added 87.2 grams ofIllinois Mineral, type 1160 amorphous diatomaceous silica powder. Theslurry was stirred and evaporated down to a thick paste. This was driedovernight at 110° C. and then ground and screened through 50 mesh.

This material was coated onto 10-20 mesh low porosity Alundum in thesame manner as described in Example 6. The finished product consisted of5% powder in the coating and 95% Alundum support.

EXAMPLE 11 5% (6V₂ O₅.94TiO₂) and 95% Alundum

6 grams of vanadium pentoxide was slurried in 200 ml. of distilledwater. To this was added 0.64 gram of 85% hydrazine hydrate. The brownsuspension turned color to dark green and then black. The slurry wasstirred and refluxed for two hours. The slurry was then diluted to about400 ml. with distilled water and 94 grams of powdered titanium dioxide(anatase) was added. This slurry was stirred and heated under reflux forabout an hour then allowed to evaporate to a thick paste. This materialwas dried at 110° C. for three hours and a portion was ground andscreened through 60 mesh for the coating procedure.

30 grams of low porosity Norton BA 307 Alundum (10-20 mesh) was soakedin distilled water for ten minutes and then dried on a paper towel. Thewater pickup amounted to 1.2 grams. The Alundum appeared moist andsticky so it was carefully dried with a stream of air until the watercontent was reduced to 0.6 gram. It was then free flowing and outwardlydry.

In a rotating round glass jar, the Alundum was coated with the abovepowder which was added in three separate increments of 0.53 grams each.The final product was dried at 110° C. The coating was uniform with asomewhat powdery surface. It consisted of 5% active material in thecoating and 95% Alundum support.

In the same manner as described in the examples above, other reactantratios can be employed; for example air and ortho-xylene in a ratio of50/1 could be fed over a catalyst at a pressure of 2 atmospheres and atemperature of 325° C. to obtain desirable yields of phthalic anhydride.

Also in the same manner as shown for the catalysts above, othercatalysts of the invention, such as 5% (10Cu₂ Fe₀.1 B₃ V₁₂ O_(x).90TiO₂)and 95% titania spheres, 30% (2CoMo₂ V₁₂ O_(x). 98TiO_(x)) and 70%silica spheres and 10% (15Ni₂ MnW₀.5 Sb₅ V₁₂ O_(x).85SiO₂) and 90%alumina spheres could be employed as the catalysts of the invention toobtain the especially desirable results of the invention.

                                      Table                                       __________________________________________________________________________    Oxidation of Ortho-Xylene to Phthalic Anhydride                                                                          Per Pass                                                                      Conversion, %                                            Temp., ° C.                                                                         Air/                                                                              C.T.                                                                              Phthalic                                                                            Maleic                       Example                                                                              Catalyst       %V  Bath                                                                              Exotherm                                                                           Xylene                                                                            Sec.                                                                              Anhydride                                                                           Anhydride                    __________________________________________________________________________    1    20.6% (Sb.sub.2 B.sub.3 V.sub.12 O.sub.x +W° 1.32)                                      6.95                                                                              357 377   69 1.1 35.4  8.7                               and 79.4% Alundum                                                        2    9.7% (Sb.sub.2 B.sub.3 V.sub.12 O.sub.x +W° 0.6)                                        3.53                                                                              372 385   69 1.1 45.8  3.8                               and 90.3% Alundum                                                        3      "              "   385 472   69 1.0 43.8  9.7                          4    5% (6B.sub.3 V.sub.12 O.sub.x · 94TiO.sub.2)                                          0.14                                                                              399 426   69 1.0 66.3  5.2                               and 95% Alundum                                                          5      "              "   399 419  100 1.0 73.8  5.3                          6    5% (6Sb.sub.2 B.sub.3 V.sub.12 O.sub.x · 94TiO.sub.2)                                 0.12                                                                              399 417  100 1.0 75.0  6.6                               and 95% Alundum                                                          7      "              "   385 393  100 1.0 77.7  4.4                          8    8% (Sb.sub.2 B.sub.3 V.sub.12 O.sub.x)                                                         3.12                                                                              399 459  100 1.0 47.2  10.5                              and 92% Alundum                                                          9    5% (6Sb.sub.2 B.sub.3 V.sub.12 O.sub.x · 94ZrO.sub.2                                  0.12)                                                                             455 472  100 1.0  58.8*                                  and 95% Alundum                                                          10   5% (6Sb.sub.2 B.sub.3 V.sub.12 O.sub.x · 94SiO.sub.2)                                 "   427 443  100 1.0 66.8  4.9                               and 95% Alundum                                                          11   5% (6V.sub.2 O.sub.5 · 94TiO.sub.2)                                                   0.17                                                                              399 416  100 1.0 64.1  7.1                               and 95% Alundum                                                          __________________________________________________________________________     *Measured as total acid                                                  

We claim:
 1. In the process for preparing phthalic anhydride bycontacting a mixture of ortho-xylene and molecular oxygen at atemperature of about 200° to about 600° C. in the presence of anoxidation catalyst, the improvement comprisingusing an oxidationcatalyst consisting of:a. an essentially inert, at least partiallyporous support having a particle size of at least about 20 microns, saidinert support having an outer surface; and b. a coating of acatalytically active material on said outer surface of said supportwhich strongly adheres to said outer surface of said inert support, saidcatalytically active material containing boron, antimony or mixturethereof and an oxide of vanadium such that the weight of vanadium isless than 10% of the total weight of the oxidation catalyst, and whereinthe active material optionally contains silica.
 2. The process of claim1 wherein the catalyst contains less than about 5% by weight ofvanadium.
 3. The process of claim 1 wherein the catalyst contains lessthan about 1% by weight of vanadium.
 4. The process of claim 1 whereinthe catalytically active material contains

    D.sub.b V.sub.12 O.sub.x

wherein D is B, Sb or mixture thereof; b is a number from greater than 0to 10; and x is the number of oxygens required to satisfy the valencerequirements of the other elements present.
 5. The process of claim 4wherein b is a number greater than 0 to
 5. 6. The process of claim 1wherein the inert support has a particle size of about 0.2 cm. to about2 cm.
 7. The process of claim 1 wherein the reaction is conductedbetween about 300° and about 500° C.
 8. The process of claim 1 whereinthe catalytically active material consists of a mixture of the activecatalytic ingredients and silica.
 9. The process of claim 8 wherein theoxidation catalyst contains less than about 0.5% by weight of vanadium.10. The process of claim 1 wherein the catalyst is prepared by (1)contacting said inert support with a measured amount of liquid toproduce a partially wet support, said partially wet support being onethat does not have the appearance of having liquid on the outer surfaceof the support, but has at least some liquid absorbed on the support,and (2) rolling the support in a powder of the catalytically activematerial to produce a support having a uniform coating of thecatalytically active material on the outer surface of said support. 11.The process of claim 10 wherein the liquid employed is water.
 12. Theprocess of claim 10 wherein the support is capable of absorbing at least1% by weight water based on the weight of the support.